Alloderm can be used at any time, but I am concerned that you think it will make a difference in your comfort.
I do not believe placing Alloderm will make ANY difference in your discomfort...
Without knowing more, it is impossible to know what IS causing your pain, but in most cases like yours, pain is caused by tightening of the implant capsule (contraction) your body has formed around the expander. Because the expender is so rigid, and because this scar tissue often connects with the scar in your axillae (from the lymph node dissection), tightening of the scar tissue can cause significant pain and often does radiate into the armpit.
There is no reason that Alloderm, in and of itself, will improve this.
I DO think that careful release of your capsules at the time of surgery and appropriate capsular management in the postoperative period will improve this however.
If you need Alloderm to improve the aesthetic outcome of your breast reconstruction, that's one thing- if what you are looking for is comfort, it probably won't help as much as what I describe above.