I believe all you have to do is this:

longvar=wordvar
IF wordvar.15 THEN longvar.HIGHWORD=$FFFF

The second line simply fills in the high word of the long with 1's if the number is negative. This is needed because bit 31 must be 1 to indicate that its negative, and because negative numbers are represented in 2's compliment form, the extra bits must be 1's.

Note: untested