Hi!
 
A bit of a newbie questions as I haven't used hardware much.
 
I recently bought myself a couple of hardware synths. I connect them via their analog out into my Motu Ultralite MK3 Hybrid interface. (6/8 analog IN/OUT) and can record easy into SPlat. This work very well. 
http://motu.com/products/motuaudio/ultralite-mk3/i-o.html 
 
However, my analog inputs are all used now, but I got a total number of 10 IN/14 OUT (with S/PDIF). If I want to connect more analog outputs into my Motu audio interface via the digital port, what do I need? An extra converter/box of some sort?
 
Same question apply to the following. I am a bit interested in the newly released Apollo Twin 2 from UAD. It has 2 Mic/Line input as well a optical IN. How do you connect 6 analog output into that card? 
http://www.uaudio.com/audio-interfaces/apollo-twin-mkii.html 

The converter boxes aren't cheap.  Unless you can find a great deal on one, it would be overkill for your needs.
 
One solution would be a small mixer with s/pdif out like the Edirol M10DX or M16DX. Both are discontinued, but come up for sale often, like here (though still not cheap):
 
https://reverb.com/item/3292883-edirol-m-16dx-digital-mixer
 
The M10DX or M10MX sell for about half that but are pretty rare (especially the latter model).
 
Another option, much less costly, is to install a patch bay. So instead of permanent connections, just easier access for you to patch your ins and outs.

EricDeluxe
Hi!
 
A bit of a newbie questions as I haven't used hardware much.
 
I recently bought myself a couple of hardware synths. I connect them via their analog out into my Motu Ultralite MK3 Hybrid interface. (6/8 analog IN/OUT) and can record easy into SPlat. This work very well. 
http://motu.com/products/motuaudio/ultralite-mk3/i-o.html 
 
However, my analog inputs are all used now, but I got a total number of 10 IN/14 OUT (with S/PDIF). If I want to connect more analog outputs into my Motu audio interface via the digital port, what do I need? An extra converter/box of some sort?
 
Same question apply to the following. I am a bit interested in the newly released Apollo Twin 2 from UAD. It has 2 Mic/Line input as well a optical IN. How do you connect 6 analog output into that card? 
http://www.uaudio.com/audio-interfaces/apollo-twin-mkii.html 

To add more analogue inputs to the UAD the best approach would be to use an ADAT unit via the optical in. ADAT units basically consist of a bunch of preamps and a DAC, then their output connects to the interface's optical input. Note that ADAT isn't the same thing as stereo SP/DIF, ADAT can handle more channels, the total amount usually depends on the sample rate that's being used and the capabilities of the main interface and the ADAT unit.
 
The Focusrite Octopre range are decent quality ADAT units at a price that isn't outrageous.
 
For the MOTU my approach would be either to use a patch bay or to add a mixer with the mixer's analogue outputs running into the MOTU line inputs. It could be done by SP/DIF but my personal preference would be a mixer/patchbay for simplicity. Not using the SP/DIF also allows a mixer's auxiliary channels to be routed to the audio interface's line inputs while SP/DIF is stereo only.

Accidental double-post, so content deleted.

Possibly the least expensive route is a small mixer. I do this with a small format Yamaha mixer that cost me under $300.  I used to use my Yamaha 01V but it takes up too much room. 
Here's one for $79 https://www.sweetwater.com/store/detail/1202
 
You use the mixer as a patch bay keeping all your outboard gear plugged in. 
 

Drawback to some small mixers is that they will add noise. They are likely to be the weak link in your signal chain. It might not make a difference in the mix depending on what you are recording.