It really depends on what is in the data you have. I have built bridges and converted data from one social site to another in the past, but not to oxwall (yet). So im not sure what all would be required, there are always surprises.
I dont think it would be that hard but again every time i do it i have found there to be little surprises that i didnt know or think about.
Basically you would just have to build a query to take the old data and push it into a new query that would use the new table names and field names. Remember there is more to this than just the user table.
Probably the easiest way would be to have a temp DB set up with just the oxwall structure and no data. Then build a script to grab one record from the old DB, move the data into the new tables and field names (code wise) and then open the connection to the temp DB and store the data. Then export the data into a sql file and import it into the real Oxwall DB.
Just do one record, that way if you need to tweek it or remove it then it will be easy to do. Once you get the new record working on the real Oxwall DB then you can change your counter to grab 50 or so at a time.
Another way is to add a user to Oxwall via registration, and then look thru the tables in the Oxwall DB and see where the data is for that user. Once you have that then you basically have the blueprint for what you need in your sql import.
Hope that helps.