I assume these are one-on-one matchups, i.e. you are not assigning
partners as well as opponents.
It helps to know how a round robin works before changing anything. One person (team) is outside the "loop" and plays everyone as they come to him. Everyone else is seated around a long table and plays the person across from them for each round, colors or home/away status are permanently assigned at every position. Everyone in the loop moves one position for the next round. To let two more players or teams in, increase the length of the table by one and splice in the new teams.
13 or 14 players
Rd Pairings
1: 1-14 8-7 13-2 9-6 12-3 10-5 11-4
2: 14-8 2-1 7-9 3-13 6-10 4-12 5-11
3: 2-14 9-8 1-3 10-7 13-4 11-6 12-5
4: 14-9 3-2 8-10 4-1 7-11 5-13 6-12
5: 3-14 10-9 2-4 11-8 1-5 12-7 13-6
6: 14-10 4-3 9-11 5-2 8-12 6-1 7-13
7: 4-14 11-10 3-5 12-9 2-6 13-8 1-7
8: 14-11 5-4 10-12 6-3 9-13 7-2 8-1
9: 5-14 12-11 4-6 13-10 3-7 1-9 2-8
10: 14-12 6-5 11-13 7-4 10-1 8-3 9-2
11: 6-14 13-12 5-7 1-11 4-8 2-10 3-9
12: 14-13 7-6 12-1 8-5 11-2 9-4 10-3
13: 7-14 1-13 6-8 2-12 5-9 3-11 4-10
The home team is to the left. Assign pairing numbers IAW the first round games which were actually played and the new teams will be 4 and 11 and will have to make up the contest between them.
For more information or to keep the existing schedule as far as possible, visit
late.htm. This was written for chess, but White/Black can be converted to Home/Away for almost any sport.
