Straight up 16ths? You can't. Four 16th notes to a beat. That's the math of music.
However, you can set your resolution to 32nd notes and insert three 16ths and two 32nds.
Or....
You can also use triplets and depending of course on the rhythm you are seeking.... you can place the 32nd notes in up to 8 places per beat.... or you can use 64ths and you have 16 possible places.
Depending on the tempo, this may not appear to the listener as short 32nds or shorter 64ths and it sounds like it's intended.
HOWEVER....
This may be a better alternative. If you can play a keyboard, this is probably the absolute best way to get this done. If you can actually play that section of music on a midi keyboard and use the click track to stay on time..... the software will interpret exactly what you played and put the appropriate notes exactly where you played them and assign the proper value to that note so that the playback will sound exactly like it should.