Hi,
Smart form is actual a function and this function can received table type .
So in your case you get the selected rows from the ALV and create a new internal table this table will be an exported to the smart form.
This may be helpful:
http://scn.sap.com/community/abap/blog/2013/10/22/deeper-dive-into-deep-structure--part-3
If you use cl_salv_table then look at program SALV_DEMO_TABLE_SELECTIONS
Regards.