Add nodes instead of replacing

[jaimeatstariongroup]

Prerequisites

• I have written a descriptive issue title
• I have verified that I am running the latest version of the UI-DSM
• I have searched open and closed issues to ensure it has not already been reported

Description

The nodes of the diagram needs to be added when a user clicks on them instead of replaced.

Steps to Reproduce

System Configuration

• UI-DSM version:
  • UI-DSM:
  • Other:
• Environment (Operating system, version and so on):
• .NET Framework version:
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