Two concerns:
1. Your final volume will increase, decreasing your OG.
2. You aren't boiling your 2nd mash which would be a big mistake.
How much grain did you forget and was it base malt or an adjunct? If you forgot a small amount of base malt I would say **** it and leave it as is. If you forgot an adjunct I would do a mini-mash and boil it for 5-10 min before cooling and adding it to the fermentor. If you forgot a substantial amount of base malts and you already mashed+sparged at full volume.... I don't know what to tell you.