The easiest way is to get the list of symbols from the other screener databases into a space or comma seperated list. Then paste that into Finviz's screener and then save that into a portfolio. You will lose the old info, like the date you purchased and the price you paid.
Come to think of it, this would be a nice feature to have, where finviz can import other portfolios. They can specify how the csv files should be formatted, like the columns needed etc.