Multiple Data Validation in one cell

Perhaps you could take advantage of Data Validation's error checking features:

Try this
• Put your list of levels in a named range...I used: LU_Level
• Select the cells to be impacted with the DV
• CTRL+1....(to open the formatting window)
...Number_tab
...Category: Text
...Click:OK
• Data.Data_Validation.Data_Validation
...Allow: List
...Source: LU_Levels
...Error_Alert_tab
...Style: Information
...UNcheck: Show error alert
...Click: OK

Now...Each cell will display a dropdown list of the levels...BUT...you can also enter anything you want.
If that works for you, we'll need to figure out how to summarize/average/whatever the resulting values.
You may need to amend your list to something like:
L -1
L 1
L 1+
etc