@ElT
It is possibly much easier to do what is needed working within S3D rather than importing.
If you know, or can measure the dimensions/radii of the corner arcs/curves and define the profile of the ‘seal/gasket’, then the following will be easy.
Use Sketch > Rectangle to create the centre line of the ‘seal/gasket’
Still in Sketch, if appropriate use Circle to create the corners, then use Trim to remove unwanted Lines, including the three remaining:
Next use Add > Construction Plane > choose option Perpendicular to Curve at Point
Tap on the Plane created using a Finger then Sketch the Profile/Cross Section of the ‘seal/gasket’ centred on the centre line already created, When the Profile is completed Hide the Plane using the Items feature to the left [Tap on the rightmost Icon in the bottom left Menu]:
Then, see above, use Tools > Sweep to form the ‘seal/gasket’, tap on the next line until it is completed:
Then hide the Sketch Planes to reveal just the ‘seal/gasket’.