I don't know, and you don't say, what to do if there's not a populated HomeFolder property. My best guess would be something like this:
$DefaultHomeFolderLocation = '\\disk1\homefolder'
$u = Get-ADUser -Id useracct -properties HomeDirectory
if ($u.HomeDirectory){
$newHomeFolderPath = "$(Split-Path $u.HomeDirectory -Parent)" + "\$userName"
}
else{
$newHomeFolderPath = $DefaultHomeFolderLocation + "\$userName"
}