I'm looking at trying to use a digital piano with only a MIDI out (no 'in' or 'through') to play some of my soft synths in Sonar. Is this possible or do I need a keyboard with a MIDI in as well?

I have an Alesis MIDI control hub/ USB interface if that is useful

You would only need MIDI In if you want Sonar or other software / hardware to control your piano.
It should work fine, provided you already have a MIDI interface on your computer to accept the external MIDI signal from the piano.
(I'm assuming the piano has a 5-pin MIDI DIN connection, not USB MIDI).

 
Should work fine.  Connect a midi cable from the "midi out" on your keyboard to the "midi in" on the Alesis interface.  Then a usb cable between the Alesis and your computer.  Open Sonar and in "Edit / Preferences / Midi / Devices" you should see your Alesis in the "Inputs" list.  Check its box.
 
Then read https://www.cakewalk.com/Documentation?product=SONAR&language=3&help=Beginners.03.html
 

Thanks a ton guys!!

Yes it will work fine.
 
I have my Yamaha AN1X hooked up with only the Midi out connected to my interface, even though it has a dedicated Midi in