Is 12 GB supported in the specs, no. Will it work maybe but again it is not a supported configuration. And always a much better idea to have identical RAM modules installed (2 x4, 2 x 2, 2 x 8, etc.). The supported totals are 2, 4, 6, 8, and 16 GB. The service manual with specs is here.

In most cases memory needs to be the same brand/type.  Mixing memory usually results in memory errors and even non booting.

You can try what you want but be prepared for problems.

Here is the Crucial Memory listing for your machine. Crucial guarantees their memory to work in a Dell (provided its all Crucial).

https://www.crucial.com/usa/en/compatible-upgrade-for/Dell/optiplex-7010-%28small-form-factor%29

I know, i know... "it's not in the system specs...",... but that doesn't mean it won't work.

For instance, in the specs they only speak about a few graphic cards... but everyone knows that there're a lot of different graphic cards which works.

Maybe is the same with the RAM, that's why asked, just to know if anyone has tried it before.

If nobody has tried it before and can share its experience, then i will simply buy another module of 4GB (of same speed, voltage, and latency).

Thanks anyway!

@JBertoMS 

Most likely it will work.

I can't guarantee this, as I don't have your exact 7010 sff sitting here on my table. But It works with even older dell machines. It is a matter of the bios, and mostly the memory controller on the intel chipset.

There is a lot of lore around dell campfires, mostly old wives tales, and sometimes, dell's original sales options become chiselled into the Holy dell scriptures, which are holy tablets of course, and we must not disobey. I guess dell is as much a religion as apple.

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I have an old dell running three different brands of memory modules. In two different sizes. Two of them are an official holy dell taboo size (8GB), and one of these doubly taboo from  the evil kingston  manufacturer.

I have tested them all with memtest 86 and memtest 86+.

It's been running fine for quite a while.

There are exceptions but for most what we suggest on here is appropriate.  I've been on this forum since 2002 and have seen many user reported problems with mixed memory.  

Mixed memory is not just a Dell issue.  I have a home built desktop using an ASUS ROG (Gamer) motherboard.  I have 16GB (two 8 GB sticks) of Corsair memory and 16GB (two 8 GB sticks) of GSkill memory.  Both are the same speed and specs.  Either works (in any memory slot) in the ASUS motherboard but if both are installed it will not boot and shows a memory error.   

Well, finally I bought one 4GB DDR3 module of a different brand than the "official" installed in my 7010, but with same speed (1600Mhz), same latency (CL11) and same voltage (1.5v)... everything works perfect...

Whew! So you got away with actually mixing the memory brands? So did I (and size as well), but it's a very scary thing. I hear tell some even survived mixing voltages.

I reckon it is all a matter of luck and alignment of the stars and planets