there is no such thing as "obtaining" whatsapp.
there is either work by your peers here being done on a port or application ...to bring "x" to the device line..

....or not...

If you were to use a browser on the n900 which would allow you to utilize "whatsapp web" ...then whatsapp is possible.

Such a native n900 browser may or may not work ... the n900 browsers are old.
and have not been updated..(we do not have folks here who can devote the time and expertise to maintain a browser which would be current enough for modern social media...which is perpetually changing it's face...)

Possibly a browser in a version of a distro which one of the guys has been working on....may work.
Kali...
Easy Deb perhaps...
Still...it must be current browser ...with the ability of the distro to get needed updates without breaking ....
and especially needed browser updates.

Kali is a current ...almost bug free... option...
I personally think Antix would be more stable and perform better..,
but...
Making distros for arm isn't my strong suit ...

But for you and everyone else who want perpetually changing social media apps to function on the n900...
the best bet is a distro.
Easy Kali
Easy Deb.
Easy "x"

You could scream loud enough ...that someone hears and comes up with a solution .