You can import onPremisesExtensionAttributes with EPE, but it must be done with syntax onPremisesExtensionAttributes. It will bring all the onPremisesExtensionAttributes at once.

For example:

1. Create an text attribute to target template:
   
2. Map onPremisesExtensionAttributes to that attribute in EPE:
   
3. Run EPE task after the changes. Now you can see the result on onPremisesExtensionAttribute. EPE will export all sub-objects as string entries:
   
4. Use expression to get needed onPremisesExtensionAttribute information. For example extensionAttribute1:

   1. #for Jython version 2.2
      import re 
      if onPremisesExtensionAttributes: 
        m = re.search(r'\"extensionAttribute1\"(.*),\"extensionAttribute3\"', onPremisesExtensionAttributes) 
        if m: 
          _output = m.group(1) 
          if _output and not _output == ":null": 
            _result = _output[2:-1] 
          else: 
            _result = None 
        else: 
          _result = None 
      else:     
        _result = None

   2. #for Jython version 2.7 or newer
      import json 
      _result = onPremisesExtensionAttributes
      if onPremisesExtensionAttributes:
         _data = this.get("onPremisesExtensionAttributes")
         _obj = json.loads(_data)
         _value = _obj["extensionAttribute14"]
         this.set("extensionAttribute14code",_value)
      

5. Save IGA account or run EPE task again: