changing joining type of glyphs from U-JOINING to D-joining


Hi there
I have a problem with joining- type of my newly added glyph. i have added a glyph
to arabic font of which the unicode was supplied by the unicode consotium this year. its a dual joining glyph (that joins on both sides)and it has two form medial and final.its unicode is U+0620. But it does not join like other glyphs.it remains in its isolated form . i need to change its joining type from default joining type(which is non-joining or U) to dual joining( D).I dont know how to do it. i m using FONTLAB STUDIO 5.0 as my tool.
thanks in advance