No...do not use DOT 5 in any braking system not set up for it...! Ethylene Glycol DOT 3, 4 and 5.1 based brake fluids are by design supposed to swell seals, while DOT 5 systems use closer tolerance components because it does not swell seals, its' virtues being not as agroscopic (collector of water), BUT it also is more viscous (heavier) and does not "seep" as readily or mix with EG based brake fluid.
I did a little experiment, had some DOT 3 in the bottom of a bleeder container, then poured in equal amount of purple DOT 5, it just looked like large round purple globules in the amber DOT 4, and eventually settled to the bottom when left alone (after 15 minutes is all it took) for the blobs to merge and settle: purple on the bottom, amber EG based fluid floated to top. Even tried shaking container vigorously, 15 minutes later same result.