As far as I know there are no functions to export datasets in genpop format. `

You should check out the results of genind2df, which makes a table that can be tweaked to something like the genpop format. I haven't tested the below, but it should get you close enough to see what else needs doing:

gp <- genind2df(nancycats)
gp[,1] <- paste(gp[,1], ",", sep="")
gp[is.na(gp)] <- '000000'
nloc <- dim(gp)[2] -1
gp_table <- rbind( c("", paste(names(gp)[2:nloc+1], ",", sep="")), 
                   c("POP", rep("", nloc)), 
                   gp)
write.table(gp_table, "nancycats.gp", quote=FALSE, row.names=FALSE, col.names=FALSE)