No, you won't be able to run 15kHz native resolutions on your 25kHz monitor. A 25kHz monitor can only display 25kHz resolutions. You'd have to scale 15kHz resolutions up somewhat, and it won't scale very evenly. 15kHz means around 240p, which scales much better to 480p, which you'd need a 31kHz monitor for.
Are you positive your monitor is 25kHz only? You'd need a different display do get a nice picture with anything other native 25kHz games, of which there's not many good ones. Get a 15kHz monitor, a trisync 15/25/31kHz monitor, or a 15kHz "standard definition" CRT TV.