Is verbing in "I medalled in volleyball" etc correct?

You hear that usage every time you watch the Olympics. "Medalled" is very much a verb in that community, and the announcers have picked it up wholesale.

Yes, it is grammatically correct.

It is also the most natural and concise way to state, simply, that you have been awarded a medal for an accomplishment.

I medaled at the qualifying event.
I medalled at the qualifying event.

Over twenty years ago, I asked the then head of the English Department at the secondary school where we both taught about the acceptability of various noun-to-verb conversions or claimed conversions. He said that a visiting professor at a lecture he had recently attended had declared, "Oh yes - you can verb any noun nowadays."

'She silvered in the 200' and 'She podiumed at the Nationals' were two examples claimed to have been used in an Australian newspaper a few years back. I'm waiting for 'He jugged in the Open.'

Yes, it's grammatically correct, though perhaps not very natural.

The verb medal is a relatively new introduction to the English language though, and is certainly not commonly used where I live (Britain).

See this analysis at Dictionary.com, which gives two accepted uses of medal as a verb:

–verb (used with object)
3. to decorate or honor with a medal.
–verb (used without object)
4. to receive a medal, esp. in a sporting event: He medaled in three of four races.