1. set up FORMS_PATH(in 11i,it is FORMS60_PATH), make this env parameter point to directory where pll location
FORMS_PATH is used by Forms Builder to record the location of attached libraries (.pll) and subclassed objects (such as items from a Template.fmb or Object Library [.olb]).

2.In your pld file header, write
.attach LIBRARY library_name END NOCONFIRM

3.Then in your new pld file, you can invoke procedure/function from attached Libary.

.pld file structure see below example:

.attach LIBRARY AttachedLib END NOCONFIRM

PACKAGE PACK01 IS
  Procedure Proc01;
  Procedure Proc02;
END;

PACKAGE BODY CHECK_TREE IS
 Procedure Proc01 IS
   ...
   AttachedLib.Procedure;
   ...
 END;
 
 Procedure Proc02 IS
   ...
   AttachedLib.Procedure;
   ...
 END;
END;