mediaID

Hi, just got mairlist activated and browsed the forum that seems ID for the asset cant be changed. so we are looking to automate extID creation whenever an asset has been imported. The scenario is we have different folders and want to assign the externalID for each asset based on the folder for example Folder1 named SONG, all the asset transferred to this folder will going to have SNG+ autogenerated number in series as the EXTID and so on. please advise how we can implement this.thanks