It looks like the OM_NEW or OM_SET methods import it and the OM_GET method exports it with the SGA_BitMap attribute . Though the autodocs seems unclear on getting the resultant bitmap.

Like so:
SGA_BitMap (struct BitMap *)
Pointer to the source bitmap which will be copied from
when used with OM_NEW, OM_SET and OM_UPDATE. This bitmap
is only read from.

The sketchboard object always uses its own internal bitmap
for editing. The internal bitmap can be retrieved using
this attribute and OM_GET. Set the gadget to GA_ReadOnly
before invoking OM_GET and be ready to handle a NULL
pointer if the gadget has not yet been layed out.

Defaults to NULL (blank).

Applicability is (OM_NEW, OM_SET, OM_UPDATE, OM_GET)