If you don't have the files on the ssd, then you won't have the speed of the ssd, hence you won't benifit to put it elsewhere. the whole idea to put a ssd in your computer is speed. we put the most accessed file in the ssd, so when we want them we can get them faster than traditional hard disk. The flash drives I used are way slower than the tradional hard drive, I don't know if there are other flash drive actually faster than hard drives, I haven't seem any still.
If you have enought ram, maybe you can create a ram drive then every time you turn on your computer, you copy the most used files there. then you would have a super faster access.
Best wishes