Death Guard FaQ (relativ naheliegend ^^):
Q: Can Poxwalkers use the Curse of the Walking Pox ability to
increase the unit above its starting strength?
A: Yes.
Q: If the Poxwalkers’ Curse of the Walking Pox ability increases
the unit above its starting strength in a matched play game,
does this cost reinforcement points?
A: Yes, you must pay reinforcement points for each
Poxwalker model that you add to the unit that would
increase the unit above its starting strength.