In theory you can. I did not try it myself though.
You can add more function calls (or modify existing ones) in the AutoSPInstallerMain.ps1 which is the main script that controls the flow of the entire install process. You could also change the process flow by modifying the script in this file.
Then you may add new functions in AutoSPInstallerFunctionsCustom.ps1 or if you want to modify a function that exists in in AutoSPInstallerFunctions.ps1 (i.e. override it) then copy it in AutoSPInstallerFunctionsCustom.ps1 and modify it there.
The reason the overriding works is that in AutoSPInstallerMain.ps1 the functions in AutoSPInstallerFunctionsCustom.ps1 is loaded after (overrides) the functions in AutoSPInstallerFunctions.ps1
To install project server, you will need to add the:
1) Steps to install binaries.
2) Steps to configure it. (I know something exists for Project Server 2013 at
) so, I am sure you can use that or find the code to configure Project Server 2010.
Having said that you need to weigh whether it is really a good idea to merge those 2 scripts or just create/use another script for Project Server.
Joseph, SharePoint MCM