Hi ! 

Can I rebuild Ford S- Max 5 seats to 7 seater ? 

With out knowing your skills level, I can't say if you personally can do it. If you are asking can it be done then yes, most things can be done if enough money is spent.

Good luck explaining the conversion to your insurance company.

https://en.wikipedia.org/wiki/Ford_S-Max
It looks like a Trend version?
"The Trend is basically a normal S-Max, but without the back row of seats. The idea is to make it more affordable, and to give more storage space to people who might want it."
You'd likely need a lot of (expensive?) parts if they are even still available.
My only suggestion would be to get them all from a scrap 7 seater if one available near you?

The insurance issue/any legal issues may well remain even if it is possible

I have two Ford S-Max. One is 7 seats, which can no longer be used and the second S-Max is 5 seats.
Which I want to put into use.

Both match - type : WA6 and type approval no. : e13*2001/116*0185. 
Also match Length, Width and Height

I wonder if I can just swap the interior without any problems. Whether seat belt fasteners mounting holes
and other mounting holes match ? Do they exist at all ?

I can apply for a renovation permit from the local car registry

7 seats Ford S-Max  engine code AZWA, 2.0 TDI 100 kW, year 2008, type : WA6 and type approval no. : e13*2001/116*0185

5 seats Ford S-Max engine code QXWB, 2.0 TDI 103 kW, year 2008, type : WA6 and type approval no. : e13*2001/116*0185

If the body shell is unique to 7 seats then it will be hard work converting a 5 seater to a 7 seater

If the body shells are the same then all the mounting points will be there, you will need all the upper and lower trim (not doors), seat belts, carpets and seats and, probably, the main cabin harness, airbag module, restraints module, cluster and BCM.

The curtain airbag may be different too, so you may need the headliner as well

AFAIK,  rearmost seats have no airbag coverage?

Thank you for your answers ! 

I'll try to do it. I will start next week.

I post the result here as well.

I'd look at both and check mounting points for seat belt in particular and seats for safety. Beyond that it's cosmetic?