I have been toying with this idea as an experiment.
On Vanguard when I click into conversions, I get a message that I can't convert the CD (or other fixed instrument) unless I convert your entire account. (I cannot check a box for conversion next to the fixed instruments.)
i have a small Fidelity account (traditional and Roth - which I rolled over from Vanguard) with which I have been playing. I transferred some cash as part of a 60 day roll over from Vanguard to Fidelity and one share of a Vanguard ETF into the traditional.
The way the Fidelity account appears when I pull up a transfer request - it looks as if it would transfer (or convert) partial shares of an instrument from the traditional to the Roth. So I tried that with my one ETF share - and it sold the whole thing and transferred the proceeds. (Which was not what I wanted.) I bought it again - and transferred the share. BTW - the taxable amount was the value of the ETF share at closing, which was ok as that fund was dropping like a stone on the date of the conversion.
I called CS who indicated that my account wasn't set up for partial transfers and they "fixed" it, but I haven't tried to transfer a partial share. I have a CD in my traditional IRA at Fidelity which is paying monthly interest, and I transfer/ "convert" the interest each month when it is paid.
There doesn't seem to be anything in Fidelity precluding the transfer/ conversion of fixed instruments. Maybe I will try buying a tiny fractional share of a CD and try to convert a portion or share of that at a time. (I am afraid to try to convert a portion or share of the CD that I have as I don't want to end up having it sold by mistake.)
Update - I went over to Fidelity - and it will let me convert one share of my CD.