The word here so far is that they won't... sucks for those just getting into the series.
The thing with reprints is,
if there is demand they get published. For a very long time, it was only "Mission: Gamma" #1 that was out of print and, to satisfy demand for that particular title from book shops, the second reprint omnibus went to press. Now, that omnibus didn't sell as fast as Pocket needed it to, so the future of a
next omnibus awaits a time when sufficient bookshops have tried ordering Books #3 and 4 of "Mission: Gamma", thus flagging a demand to Pocket.
There are many more second hand copies of "Mission: Gamma" #2-4 floating around than there were of #1, because they had bigger print runs in the first place. The best strategy people can do is probably tell their bookshops they wish to order the now out-of-print #3 and 4.
Marco said a third omnibus was unlikely for now, IIRC, but he didn't say unlikely forever.