Okay so, I'm not sure what you mean by converting being allowed, so let me explain both.
a) You cannot sell credits in any way, shape, or form. This is due to IMVU having to report creator income to the IRS(U.S. Agency), therefore they have their own payout system for creators now and have banned the selling of credits everywhere, and the buying of credits anywhere other than the IMVU store. Doing so will lead to your account(s) being permanently banned on IMVU.
b) The current conversion rate set by IMVU is $0.40usd per 1,000 IMVU credits. However, this information is only useful to artists who accept credits as a payment option and would like to charge the equivalent of cash.