Unless someone has better knowledge on this, I don't think this will be possible using a table. I tried using SQL Server to convert a text based date "01/01/2020" to an integer based date - 20200101 - which you can use in Incremental Refresh. However, while I can get the text to work, converting to a number breaks folding, just like converting text to a date does.
This was the code just to get the year to a number. I didn't bother with the rest as that broke folding.
I think the best approach here is to work with your DBA and create a view of the table that converts the text date to a true date, then use that for incremental refresh instead of connecting to the table.
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DAX is for Analysis. Power Query is for Data Modeling
There is a scenario of "date columns of integer surrogate keys in the form of yyyymmdd". And based on the official document, we can create a function that converts the date/time value in the parameters to match the integer surrogate key of the data source table. For detailed steps, please refer to this document: