If your league already decided which keepers, it's pretty easy, you can click on any square on the draftboard pre-draft and set players.
If your league hasn't decided on their keepers or may change their minds, we have an official keeper system. Since you are moving your league over you need to follow these steps.
The concept is to create a dynasty league to fill out your roster, add a supplemental draft, and finally change it back to keeper mode.
Step 1: Set your league to Dynasty mode
Step 2: Follow this exactly - https://support.sleeper.app/leagues/dynasty/how-to-move-my-dynasty-league-to-sleeper-from-another-platform
Step 3: Make sure after your draft you "Add Supplemental Draft"
Step 4: Only after adding Supplemental draft, change your league from Dynasty to Keeper mode
Now everyone has their full keepers from last year to choose from, and you will even have the option to choose which rounds they lose on the draftboard once they choose their keepers.