How to increase a transformer's primary voltage rating?
my only concern is whether the 250 VAC (rms) primary rated transformer
will survive high voltages (upto 400 VAC rms).

Most commercial transformers will operate close to magnetic core saturation and so any significant increase in the AC supply voltage may result in fuses blowing or breakers tripping.
Plus, the insulation rating between primary and secondary is chosen to withstand overloads based as a multiple of the peak AC handling voltage so, you are at significant risk of endangering lives by using a 250 volt rated transformer on 400 volts AC.

If not, what are the ways that can help increase the primary voltage
rating. For example, combining 2 transformers, or anything?

Use the correct transformer for the job is my advice. Putting two transformer primaries in series does not guarantee that they will share the applied voltage equally due to manufacturing tolerances in the windings (+/- 50% I've seen). You would also need to ensure that they were loaded symmetrically on the secondary.