Yes you can do this from the start page using the automated request form (sc_trans & SHOUTcast v2 only), however you may need to change the settings for your automated song requests, by default (and by law) it's set to 30 minutes, just change it to 1 and restart, then you can just type the song you want to hear next into the request form
You can also create a playlist, set the type to "Immediate", and the playback order should be "In Order"
....enable the playlist and then goto the media library and just drag the next song(s) you want to hear into the list, remember to save after each change