To add polygons to an existing cluster,
Select the polygons you want to add,
Open the Explorer, find the cluster,
Select the cluster with shift key,
Do MCP > Edit > Add to Cluster
For the texture mapping you will have to rescale the Texture Support to include the newly added polygons (if you didn't freezed the Texture_Projection).
If you edited uv coordinates in the Texture Editor, you will have to make the new sample points fit the other ones, as the sample points of the newly added polygons will probably be detached from the others. This is an example where the Heal function is useful.
Hope this helps
Salutations - Cheers
Bernard Lebel