Looking to pass values from Execution Plan set up

You can do what essentially what you're describing with Data Driven Testing.  The idea is you read data from a file (e.g. Excel sheet or CSV), in your case user names, and run the tests with each data item.  This can be done using scripts or in keyword tests.

Using Keyword Tests for Data-Driven Testing | TestComplete Documentation (smartbear.com)

I have done exactly what you are describing by reading usernames and passwords from an Excel file I created.

This tutorial should help:

Data-Driven Testing With Keyword Tests - Tutorial | TestComplete Documentation (smartbear.com)

If the test is in the Executionplan multiple times then you can set the parameter to different values.

I'm using scripts but KWT works the same way

Looks a little like this:

Just make sure the Keywordtest uses a Parameter

You can set them here:

Not sure if it is possible, I don't use the Execution Plan. May I suggest a different approach.

Let's assume you are reading from excel sheet all the user names.
On the project properties you create a variable type "data table" to load excel sheet. 
On the login script properties you create a UserName parameter.
On a top controller script you create a data loop to go thru all the sheet values, and call the login script pass to it each value.

In addition to the Data Driven Testing I should mention that parametrizing your keyword tests is another option that more closely matches what you have in mind.  Here is the help topic for that approach:

Keyword Test Parameters | TestComplete Documentation (smartbear.com)

It works more like you described.  You pass a test the login name either from another test that calls it, or directly from the Execution Plan.