You can't do what I think you're asking to do.
The only privileges you can grant on procedures are EXECUTE and DEBUG.
If you want to allow user B to create a procedure in user A schema, then user B must have the CREATE ANY PROCEDURE privilege. ALTER ANY PROCEDURE and DROP ANY PROCEDURE are the other applicable privileges required to alter or drop user A procedures for user B. All are wide ranging privileges, as it doesn't restrict user B to any particular schema. User B should be highly trusted if granted these privileges.
EDIT:
As Justin mentioned, the way to give execution rights to A for a procedure owned by B:
GRANT EXECUTE ON b.procedure_name TO a;