Sure. Start by opening the editor. The main database will load. You will need to find the team to which you want to add a reserve team.
To do that, click Database on the left, and then click Clubs, and find the club you want using a search (it's easiest to search for a club by name).
Open up the club by double-clicking it in the search results. Look for "Reserve Teams" on the left side.
This section is confusing. There's two sections, one at the top and one at the bottom.
Top section
This is for adding "fixed" reserve teams. These can have their own kits, logos, and player histories can refer to them. Players can be contracted directly to these teams. There is a hard limit of 3 of these per club.
If you add a team of this type, you will see a new section on the left, near the bottom, which you can expand and set the team's division (as well as other details).
Bottom section
This is for "FM-created" reserve teams. These can't have their own kits or logos, and players can have a contract with the main club, but not directly with these reserve teams. You can add as many as you want.
If you add a team of this type, you can set the team's division directly on this page in the table row where the team has been added.
Note - don't add 2 teams of the same type, for example 2 x B team. This will confuse the game and you will end up with one team in the game that looks like it's 2 teams.
I'd recommend you add your reserve team to an existing league and test it before attempting to go further and add a totally new league. Good luck!