The SM57 with an inline impedance transformer ought to work. This one is inexpensive but I have never used it.
http://www.sweetwater.com/store/detail/LittleIMP/

Shure actually makes one, that I have used without noticeable loss of mic signal, with interchangeable ends allowing it to be used in either direction for about $40.00. The other option is a passive direct box. They start at about $40 and the better ones are in the $100.00 range. The difference is the quality of the transformers

Using a phantom power box will not by itself bring a mic level to a line level. That is the whole point of a preamp. You will need something more akin to one of the ART single channel pre's to run two mics simultaneously.

Actually, you are in a pickle. Reading the manual, when the front inputs are active the rear inputs are not and vice versa. I don't think you are going to find a satisfactory solution for running two mic's simultaneously.

You could possibly run a mic into a passive DI ("backwards") and then 1/4" instrument level into the FW Solo. You still might not have enough gain.

This is precisely the situation I advise people to avoid when they purchase the cheapest interface they can find. The FW Solo is specifically NOT designed to do what the OP wants to do.

Sorry I thought his unit had two preamps one xlr one 1/4". Can't use one preamp with two different inputs especially at different levels. Only way to go is room mic both sources with one mic.

Thanks both for your help so far. I haven't heard of some of the equipment you suggested--the impedance transformer / the direct box / DI--and I wasn't aware that the impedance difference between the mic's output and the 1/4" input was an additional problem.

I was actually in Guitar Center today, and a salesperson gave me the impression that another possibility is to run a dynamic mic into a separate mic preamp to raise the level, and then into the 1/4" input in the FW Solo. Would this work, or would something like a DI still be needed?

At this point, though, anything I would buy would be a temporary fix. And buying additional equipment to squeeze another input out of the FW Solo, especially if it's not of satisfactory quality, would probably be a waste of money. The FW Solo has served me well for years as I generally compose electronically and record only solo vocals or instruments, but now that I am trying to become more serious about producing I am sure I will find that artists do not want to be constrained by my quite limited setup. Looks like it might soon be time for an upgrade.

Its most definitely time for an upgrade. Recordings since the 1930s generally involved more than one microphone in the mix.

Even if you purchased an external phantom power supply, you would still need to purchase a low impedance to high impedance transformer adapter for your one quarter inch input. And you certainly would not have 2 inputs that even come close in matching one another.

My suggestion, purchase yourself another inexpensive USB audio device with 2 phantom powered XLR microphone inputs. Then, you could also utilize your single input device simultaneously with another pair of microphones. However, you'll find that monitoring this will be problematic. This is a good excuse to just purchase yourself one of those 8 microphone input FireWire devices. You are looking at about a minimum of $500 US for one of those. But then you will be more prepared for recording other people's bands.

Start saving your money from your paper route.
Ms. Remy Ann David