樓主，為你查了，你取得朋友的同意，應該可以放心地替你朋友簽他的名，不用模仿筆跡。找了一段律師的答複，你的情況和這個類似：

Question: My brother just left for a trip to Nepal for several weeks of trekking (it’s on his bucket list). Before he left, he was close to signing a lease on new space for his business. Apparently, the landlord didn’t get the lease agreement to my bro before he left, even though all the terms had been agreed to, so he called me from the airport to ask me to sign the lease for him when it comes in to his office. Can I legally sign the lease for him or will I be committing forgery?

Forgery

Under the law, a person commits forgery when she alters a legally significant document in a manner intended to defraud another person. A legally significant document is one that affects legal rights and/or duties, a definition that certainly includes the lease in question. In the situation you describe, you have been asked to sign for someone else and would be doing so only to assist that person. You are not intending to defraud anyone nor are you doing so. Thus, you would not be committing fraud by signing the lease on behalf of your brother. For more information about forgery in general, see our article “Forgery Laws and Penalties.”

Be aware, however, that there is a particular way that this signing should be done to make clear that you are signing with your brother’s express authority.

更多解釋：

http://www.criminaldefenselawyer.com/resources/if-someone-gives-me-permission-sign-their-name-have-i

所有跟帖： 

• 太有幫助了，非常感謝！看來做一個事情的動機是很重要的。 -Hope_2007- ♂ (0 bytes) () 02/21/2016 postreply 14:49:18

• 國內的律師告訴我，他遇到過這樣的案子， -千裏一盞燈- ♂ (560 bytes) () 02/22/2016 postreply 12:52:54