Our old CSV converter site at our sister site mtgo-stats was having problems with large collections (specifically with many distinct cards), so we have put up a Beta of our new site at csv.deckedbuilder.com for converting your .csv files into .coll2 files, to be used in Decked Builder across all platforms!

Enjoy! Email me at [email protected] if you run into any trouble with it, but it’s pretty self-explanatory and worked with some gigantic collections when I tested it.