You can't use a front derailleur, or you'd need a rear mech too (which defeats the object, though probably possible). You need to find out the ratios of the hub gears you're looking at. For the Alfine, the range is 306% (which is equivalent to a 11-34T cassette range) starting with 0.53 to 1.62. Next you choose what you need as a lowest gear, and work out a chainring/sprocket combination that gets close to that. For me I need a 40" low gear minimum but ideally 35". Alfine sprockets are 18 or 20. I would find out if you can go smaller. So for me a 61T front chainring with 18T rear would to the job, giving me a 34" low gear (because that is (61/18) * 0.53 * 18.8 (wheel diameter with Kojaks)). The upper gear would then be 103". A spreadsheet is your friend here - play around with different combinations. I just knocked one up for the Alfine and it's clear that this is designed for large wheel bikes so you still need a massive chainring or a smaller-than-standard sprocket to compensate for the small wheels.