Probably this doesn't really help with your problem but I have done a similar thing. I have a form where the user must enter dimensions. I want them to be in metres but often the information is often in inches or feet & inches. I set up so that the double click event opens a query which prompts the user to enter the value in inches then does the convert.
Using querys is a messy way but it does work. I'm trying to convert the query to code right now but keep having problems with this.