If a mosfet 2N7000 is rated 60v 200mA will it work with 12v 800mA?
If the mosfet 2N7000 is rated at a maximum 60v 200mA which give us a power of 12W will it work on 12v 800mA which is a 9,6W?*


No.
You MIGHT manage this condition for a few 10's of microseconds.
Or not at all.
But, it's well outside device specifications. See below.

Semiconductor device absolute ratings are individual ratings, not a subset of a composite rating. 

When asking questions relating to specific components or products it is usually useful to provide a link to relevant documentation. A 2N7000 datasheet is here 

In the "ABSOLUTE MAXIMUM" [shouting] table on page 1 you see figures of 60 V and 200 mA continuous, 500 mA pulsed.
The related 2N7002 has continuous and pulsed values of 115 mA and 800 mA and the NDS7002A 280 and 1500 mA respectively.

Even if you can manage to squeeze 800 mA through a 2N7000, downhill with the wind behind it, figure 1. and figure 2. on the (unnumbered) page 4 of the datasheet show why it may be a bad idea.   Vds with a gate voltage of 10V (high) is typically over 1V for a power dissipation of 800 mW+.
As page 1 specifies maximum power dissipation of 400 mW at 25 C, derated above 25C, the 800 mW is far above the rated maximum.
Note that your stated figure of 9.6W, is the notional maximum switched power for the figures that you supply, and NOT the power dissipation in the 2N7000.

Finally, Figure 13 provides a fairly definitive upper limit.
This maximum safe operating area chart is somewhat more obscure that some other typical data sheet charts, but can be quite useful once understood. This shows the range of "safe" conditions which may be combined - for parameters of voltage, current and time period.
In the lower left part of the graph "DC" conditions apply. Subject to other constraints being met you can apply these combinations indefinitely.
You'll see that as voltage and current  are increased you enter regions where allowable time durations apply. In the top left hand area you'll see a dotted "RDS(ON) limit" line. This is a restriction imposed by the inability of the device to operate in the area at far top-left due to the resistance of the fully enhanced device. As current is increased Vds rises and by the time Ids = 0.5A, Vds ~= 2.5V. That's also the point of intersection with the 100 uS (microsecond) time period line. AND the graph is for a single pulse, with the junction and device at 25C (which it will not remain at for more than a few shakes), for Vgs = 1-V (ie high)  and for a single pulse. ie overall, you can't get there from here, and you have about no time to try before you have to stop.  

Note that the Vds of 2.5V in that chart is different from the Vds of > 1V that was suggested by figure 2 &amp; 3. ALL charts are "typical" (and labelled accordingly). This difference does not make total sense, but typical charts do not have to be directly comparable, and it's liable to be wise to take the worst case one when calculating what is possible.  What is "sensible" is liable to be far less again.