Yes, it's easy to do so - I did it myself with no real programming knowledge. Just find the files CIV4EraInfos.xml, Audio2DScripts.xml and AudioDefines.xml, either in the vanilla Civ IV folder or the BTS one if you have the expansions. Then just do some copypasting of the parts that relate to existing songs and replace names in the copied parts with civ 3 songs that you put in the soundtrack folder.