Yes you can charge your 2000 mah battery with the 50 ma charger, it will just take a long time. The lower the charge rate, the more inefficient the charging so it will take a lot more than 20 hours to charge it. Even charging at 0.1C (200 ma in your case) it would take 14 to 16 hours to fully charge and equalize all cells. All is not bad using the low charge rates though as your batteries will last much longer, i.e. more charge/discharge cycles.