If you mouse over the "[?]," you get the following:
Promotional Points differ from Cryptic Points in that they can only be used in Star Trek Online. Cryptic Points, however, may be used across all Cryptic Studios' games.
Thus, it is not just that they are not going to be in Zen because they only are usable in Cryptic games; technically, they are not even CP. They are promotional points and are only valid in STO, so, theoretically, they are not even usable in Champions Online, even though that is another Cryptic game and it uses CP. As such, until they change that policy, there won't be giving you Zen instead of "promotional points" usable in STO.
But, in answer to your question, since 100 Zen = 80 CP, the conversion formulas are:
Zen * .80 = CP
CP * 1.25 = Zen
Thus, if they were to give you the stipend in Zen, it would be 400 * 1.25 = 500 Zen.