If I understand your question, the mod already does that. From the information page:
When lava hasn't flowed in a while it turns into Hot Basalt. Hot Basalt keeps the same shape as the lava that formed it, and eventually cools completely. If you look closely at fully cooled basalt, you may notice subtle cracks matching the cracks that occurred during the cooling process. (Cooling times for lava and basalt cooling are both configurable.)
Your question also prompted me to add another section there:
Flowing TerrainMinecraft wasn't designed to make curved surfaces easy. Sometimes "curvy" blocks introduced by mods look different than they behave. Unfortunately, this breaks one of the core features of Minecraft that makes it such a compelling experience: the entire game world is sliced into blocks at regular 1-meter boundaries and every block can be mined and interacted with.
Basalt and lava blocks behave like normal Minecraft blocks even though they are curved in myriad ways. They always fit within a single block space and they always have collision boxes that match their visual shape. You can walk on them, target them, mine them just like any other block.
I hope that answers your question. If I didn't understand, please expand.