首先要纠正,女主持人应该是hostess,host是男主持人.
The girl under the tree is a hostess and singer.(singger前不可加a,否则就成两个人了）
The girl under the tree is a hostess,who is also a singer.(用定语丛句补充说明）
The girl under the tree is not only a hostess but also a singer.（不仅是……而且是……）
The girl is a singer as well as a hostess.( 树下的那个姑娘不仅是主持人也是歌唱家）（你那个句子本身是对的,但是,你前面说的是强调singer“她是主持人,也是歌唱家.”而你的这句话是强调host.他不仅是歌唱家而且还是主持人.）