You asked if you could attach a DI component to current RO unit. But you don't have an RO unit. You've got a tap water filter.
Another thing that doesn't get much mention, is that a 50 or 75 gpd unit has a higher rejection rate than a 100 gpd or higher unit. A higher rejection rate yields higher quality water. If you're not going to drink the RO/DI water, and use it only for water changes and topoffs, with planning a 50 gpd RO/DI unit will meet your needs.
Another thing that doesn't get much mention, is that a 50 or 75 gpd unit has a higher rejection rate than a 100 gpd or higher unit. A higher rejection rate yields higher quality water. If you're not going to drink the RO/DI water, and use it only for water changes and topoffs, with planning a 50 gpd RO/DI unit will meet your needs.