Yes, it's simple, and you don't need an outboard mixer.
First, it's recommended that you read the Tascam manual, over & over if necessary, until you understand it. It will make a lot of these procedures easier, and make a lot more sense.
Basically, to combine multiple input sources to one track, you'd PAN [both] your input sources to the same side, either hard LEFT or hard RIGHT, and record with REC FUNCTION in BUSS L/R mode, on the corresponding BUSS, [either L or R].
->BUSS L signals get recorded onto tape tracks 1 & 3.
->BUSS R signals get recorded onto tape tracks 2 & 4.
-Use a combination of PAN and REC FUNCTION/BUSS L-R, to assign any of the mixer's inputs to any of the 4 tape tracks.
-That's the basis of what BUSS mode recording is all about, as opposed to DIRECT mode recording, which is probably for another post.
-The manual has all relevant info.