Add 'Write-in Product' to Custom Opportunity Product Sub-grid

In order to fix other functionality issues we replaced the default sub-grid with a custom sub grid for Opportunity Products within the Opportunity.

The problem is, now the only option when selecting the '+' is for an 'Existing Product'.  We want the option of also adding a 'Write-in Product' with the inline edit feature.

Any idea how to add this in?