Not sure I’ve completely understood your question but this information below may help.
It’s possible to show a page/tooltip when hovering over a datagrid column, which you mention your team wants to do for your progress column.
Within the tooltip page you would use a dataview of the entity and can show then some text with the attribute for your sub- status. Then on the datagrid properties select the tooltip page, and for the column you want it to appear you have to enable column properties to show tooltip.
There isn’t a way to limit the tooltip page to just certain rows with a certain attribute value but maybe it’s be suitable for you to show the status attribute and then the sub-status so you don’t have a blank tooltip page for those without a sub-status.
Here is tooltips in the documentation: https://docs.mendix.com/refguide8/data-grid#tooltip-page
Column properties: Grid Columns - Studio Pro 8 Guide | Mendix Documentation
If you add a new column for your sub-status attribute, as you also mention as an option, then the column will show empty unless the attribute is set so when changing your main status attribute to something without a sub-status then you can set the sub-status attribute to empty again.