DEF input and output frim Virtuoso

When streaming in (and out) def from Virtuoso I need to be able to map a cell name based on the maskshift property in def.

EXAMPLE 1:

COMPONENT 1;

- - U7 INV + PLACED ( 2000 2000 ) FS ;

END COMPONENTS

EXAMPLE 2:

COMPONENT 1;

 - U7 INV + PLACED ( 2000 2000 ) FS + MASKSHIFT 1 ;

END COMPONENTS

When reading in the def the EXAMPLE 1 would use the INV cell but the example 2 should map to the INV_MASK2 cell.  The same process needs to act in reverse when doing def out.

Can this be handled with the "User Skill File" referenced in defin/out?  The documentation for the User Skill File is lacking in the manuals I've found.  Are there any detailed description of how this works?