Hello DDK, If the ferrule fits, yes, this is method that you can get a nicely finished crimp. Some crimping tools can give the right squeeze, but leave a mark at the seam. You can definitely drill out the pipe stop, but don't drill out the ID of the part of the union that the ferrule is forced against.
You can also drill out the nut to help get it on the line, but then you loose some of the seat that pushes the ferrule and gives you a nice finish. If you drilled out the nut large enough to pass over the ferrule, but with enough edge to still hold a washer, maybe a washer could be used as the sacrificial piece. You pass the line through the Nut, Washer, Ferrule, and then through the coupling. Make up the fitting to get the desired crimp and when you take it apart, cut the washer with tin snips, and slide the nut off over the ferrule.
Pat