we can automate As 400 application using QTP.
Using Terminal Emulators(TE) user can automate As 400 application with QTP. It depends on the terminal Emulator that  u r using to access As 400 application. terminal Emulator is a third party tool to be used to access IBM I-series applications like Mainframes, As 400 etc.

QTP suupports HummingBird, Attachmate, IBM, NetManage, etc..

To work with these TE using QTP, u need to install TE Addin for QTP which is avaialable at HP site. 
based on the TE & QTP verison type, you can work with these applications. 
QTP 9.5 verison contains builtin addins but you need to select it at the time of installation(Choose Custom instead of Typical ans select the Addins required). 
After installation it ask you make configuration settings via Wizard. Select the TE type and select the Verison and give the co-ordinate values of you TE screen and complete the process so that from next time u will able to access with TE objects. make sure that TE addin is selected from Addin Manager window.

you need to  make some settings in your TE Global Options as well if your TE is HummingBird to qork with TE Objects using QTP.

TE Objects:
TeWindow
TeScreen
TeTextScreen
TeField

E.g. TeWindow("screen1342").TeScreen("SignIn").TeField("Username(protected").Set "amarendra kothuru"

It is a bit diffucult process and working with TE is also a big challenge for the QTP automation testers. 
All the best...