You may use a backquote expression—see Macro Arguments—to force the unary negation/minus operator. E.g.,
<<add _i `-_j`>>
That said, and assuming this isn’t a toy scenario, if you know the status of the second operand to know that you need to use the unary negation/minus operator in the first place, it seems like it would make more sense to have a <<sub>> widget—e.g., <<sub _i _j>>.