The following shows how to set the $HostType for the BizTalk Powershell Extensions.

 if ($HostType -eq "Isolated") 
   {
     $HostTypeCode = 2  #Isolated 
   }
   else 
   {
     $HostTypeCode = 1  #In-Process
   }
 $temp = New-Item -path $hostName -HostType:$HostTypeCode -NtGroupName:$myNTHostGroupName -AuthTrusted:$AuthTrusted 

See full function in this blog about Adding BizTalk Hosts and Hostinstances with Powershell.