You can enable home sharing, but there really is no such thing as a "shared" account. Everything is tied to your Apple ID.
If any of the music files are "protected" files (purchased from the iTunes Store before mid-2009) you can upgrade them to iTunes Plus first. Then you would be able to copy the files to her account.
John is correct, there is no true "sharing" it's more like right of use, but she can still use all the stuff she has from your account on her device, it's just a little tricky. My daughter has had her own account for over a year but she had used my account before that. She has her own iPod, Macbook Pro, and now she just bought her own iPhone.
It had never been a big deal to her until she got her phone because she has to use only her account, my e-mail and stuff she does not need on her phone. So use Home Sharing on your computer with her login and her login on her device. The biggest question is are you using two computers or one? With just a single computer it may make it harder because you really have no way of home sharing (I may stand correctly by someone else though). With two computers you just setup home sharing, login to both iTunes accounts with her login, use her computer (or the second computer) and drag all the stuff over to that iTunes library, music, apps, etc. Then they will be on her computer. Then she just drags them on to her iOS device. They will copy and she can use them.
There is no way around updates though. They can only be updated with the original account, which I believe why John stated use iTunes Plus first, because the will not play at all under a different account login. The only advantage that my daughter wanted to use this for was progress on games, so she wouldn't have to start over. Most of our stuff are free downloads so it was not even a repurchase issue as much as time on a game issue.
If you use only one computer, it should work the same way but I have not tried it so I can not say for sure. Logout of you account name and drag the stuff to her iOS device.