Yes, that is possible. You need to make sure that the first bitmap is in a format capable of holding alpha information. You could for example just set a completely translucent alpha channel for the first bitmap and overwrite that later with the data from the second bitmap.

A more straightforward approach is to create a third bitmap with bmap_create and fill it with the rgb values from the first bitmap and the a values from the second one.

In both approaches you could just iterate over the bitmap's pixels and use http://www.conitec.net/beta/apixel_for_bmap.htm and http://www.conitec.net/beta/apixel_to_bmap.htm.