come over后面为什么要加个to,还有Come over here.here是个副词，前面能用come over吗？

come over是固定词组, 加上to表示方向,去处,
是的,可以用come over.
英语中有些动词词组相当于动词come over 就相当于 go
come over to 就相当于 go to

1.come over v. 过来.抓住

I guess I could come over.
我想我能来。

2. come over to
从一个地方来到另一个地方,过来

He has come over to China for the summer.
他不远万里来中国度暑假。