Yes, I would add temporary memory to XCS the same way as proposed in my ZCS paper (Wilson, 1994). That is, add a one- or two-bit internal register to the system. Also add classifier actions capable of setting and clearing bits in the register. Finally, add corresponding bit positions in the classifier condition.
Such an addition of an internal register has been confirmed in a paper by Cliff and Ross (Adaptive Behavior, vol. 3, no. 2, pp. 101-150, 1994). (Dave Cliff, firstname.lastname@example.org). They modified ZCS with one- and two-bit registers. The program was able to learn McCallum's maze, and a more complicated variant of that maze. These are environments that require temporary memory for solution. Without the register, ZCS did not make progress. However, a fascinating result was that ZCS did rather well with just a one-bit register on a problem that in principle requires a two-bit register!
Can I add temporary memory to XCS, as you proposed doing in your paper on ZCS?