In our leagues, you can add new players at any time during the season. They just have to be registered for the team before they go on court. You can be courtside, register on your smartphone, grab your racket and jump on court.
If the new player is self rated and plays three strong computer rated 4.0 players and wins 6 0, 6 1 or 6 2 he is in danger of being disqualified. In turn all three of his matches would be written off as losses. A strong 4.0 should beat a weak 4.0 so he might not get disqualified with the same scores. If the matches are 6 3, 6 3, they are still considered the same level and he shouldn't get DQ'd but you never can count on that.
If the player lies on his self rating, he should get disqualified at the lower level, his matches will be disqualified and he will be suspended from play for a year. If the captain fills out the self rating and lies, they will both be disqualified and suspended.