ToString in Formula Column Field

I have a report I've created and there are two text columns in this report which are custom fields of type pick list.

I want to add a third "Formula Column" to this report to reflect some legacy reporting needs.  Ideally, I don't want to add a custom field for this as the only time this information needs to be accessed is for reporting.

I created a formula for this column that looks like this (real field names obfuscated for security purposes):

If(Contains($C_CustomField2,'SomeString',FALSE),"SomeString",$C_CustomField1)

The way I interpret this is the third column should always have the value of CustomField1 unless CustomField2 contains "SomeString" in which case third column should instead contain "SomeString."

This formula doesn't work because it says Contains function's first argument must be a string and CustomField2 is a field of type Pick list.  I would like to wrap this field value in the "ToString" function but I found that this function cannot be used within the context of a Formula Column.

Is there another way to achieve what I'm trying to achieve?  Ideally without creating a new custom field?

Alternatively, is there a way to make the "ToString" function available within Formula Columns in reports?  

I saw a thread requesting adding more functions to reporting from 2015 (see "Allow missing functions Formula Columns in Reports" but I'm not sure what the progress was on that.

Thanks in advance for your help!