That’s something on our list, probably right behind being able to sample the other particles in a stream instead of just the current one. I’ve avoided it so far because its really easy to eat up all your memory when loading all the particles multiple times like this.
This can be achieved currently though, by making two PRT Loaders. The main one loads the “current” frame’s particles, and the other loader has the same files but uses the frame offset to load a previous (or future) frame. Then a Magma modifier on the main loader can refer to the particles of the second loader using existing operators but it is in effect sampling a previous time.