You can top off with any Dot 3 or 4. Just reduces the benefit of using SRF. True, I top off with SRF unless it forces me to open a $75 bottle just to use a wee little bit. That's when I just use some Motul600 or regular ATE.
2015 F34 328iX GT Glacier Silver
2011 E90 M3 Monte Carlo Blue
2004 E46 M3 Imola Red