one way attached below. I´ve exploded the stl mesh and created a planar curve for the heart shape made from one box and the blends with the mirror side. Then just used _Patch (32x32, Stiffness=1) to built the upper bulged surface.
after exploding the STL, i patched the upper mesh which has the bulge, the lower mesh is planar and does not require the patch. If you patch the upper mesh this gives you a large untrimmed surface as a result. To get the heart shape, i´ve duplicated the border of the lower mesh part (the one in your picture) and created smooth curves by rebuilding parts of it, then created the smaller radius blends. After mirroring one side of the heart, i´ve joined to one closed curve.
If you extrude this curve this will give the wall surface which can be used to trim/split with the upper bulged surface.
Hi
Use the @clement approach but before creating patch, create a profile curve that have this little bulge. Then use this curve with border curve to create patch.
After creating the patch, move it down a little and then trim it using walls of the heart, and trim the walls using created patch. This way you will be able to join everything together and apply fillet etc.
Usually (90% of the time) patch surface can’t be joined with other since it is not that exact, I’am not sure if this is even possible to create surface that looks good and sticks exactly to the borders.