Solved it: Using the doc.SendStringToExecute method.
Source: start lisp-Command immediately after sendstringtoexecute - Autodesk Community
# Load the Python Standard and DesignScript Libraries
import sys
import clr
# Add Assemblies for AutoCAD and Civil3D
clr.AddReference('AcMgd')
clr.AddReference('AcCoreMgd')
clr.AddReference('AcDbMgd')
clr.AddReference('AecBaseMgd')
clr.AddReference('AecPropDataMgd')
clr.AddReference('AeccDbMgd')
# Import references from AutoCAD
from Autodesk.AutoCAD.Runtime import *
from Autodesk.AutoCAD.ApplicationServices import *
from Autodesk.AutoCAD.EditorInput import *
from Autodesk.AutoCAD.DatabaseServices import *
from Autodesk.AutoCAD.Geometry import *
# Import references from Civil3D
from Autodesk.Civil.ApplicationServices import *
from Autodesk.Civil.DatabaseServices import *
# The inputs to this node will be stored as a list in the IN variables.
dataEnteringNode = IN
adoc = Application.DocumentManager.MdiActiveDocument
editor = adoc.Editor
def load_and_start_lisp(LispFilePath,LispCommand):
Lisp2Load="(load " + "\"" + LispFilePath.Replace("\\","\\\\") + "\"" + ") "
adoc.SendStringToExecute(Lisp2Load,True,False,False)
if LispCommand != "":
adoc.SendStringToExecute(LispCommand.Trim() + " ", True,False,False)
return Lisp2Load
# Assign your output to the OUT variable.
OUT = load_and_start_lisp(IN[0],IN[1])