Electrical Dummy's Turntable Question...

I've got an MMF-5 that I've been recording LPs to iTunes with via a BBE FJB-200X phono preamp and RCA into USB cord. The grounding post on the phono preamp is just a wee bit too big for the connecting spade thing at the end of the MMF-5's ground wire and it's unwieldy to improvise this ground connection (plus I don't want to damage the turntable's ground wire/connector). Is it possible for me to just leave the MMF-5's ground wire hooked up to the ground connection on my receiver while running the RCA left and rights thru the BBE phono preamp without doing any harm?

There's no way to know from an objective standpoint . . . this depends on the design of the grounding wires and traces inside both your receiver and the phono preamp, and the resistance of the ground conductors in your interconnects and their connectors.

But there's no harm in trying it . . . and if you get a hum-free connections, you're fine. If not, you can make an adaptor with a short piece of wire, and a machine screw, nut, and washer(s) to make the connection between the wire and grounding lead (fork) of your turntable - this should leave the turntable leads completely unmolested. If you're a perfectionist, use brass or stainless-steel hardware, and wrap it in electrical tape or heat-shrink tubing to prevent errant shorts.

Also, the output cable of the MMF-5 IIRC is somewhat sensitive to magnetic coupling, meaning that if you get a bit of hum, try moving the cable around to minimise it.