is it possible?

No. Even assuming that the 2 objects are Attached (elements of a single Editable Poly object) that can’t be done - you can only weld vertices where the edge between them is Open (has a poly on only one side).
That would be true for the Plane element, but not the Box element as all its edges have polys on both sides.
You would have to delete the poly facing the Plane element in order to weld them. The highlighted poly in the image is the one to remove.

Max 4.2 through 2013.
XP-64 (SP2)
NVidia 9800GTX-512 (Driver 266.58).
Core 2 Quad Q6600 2.4GHz, 8Gb Ram, DX9.0c.

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In other words: one cannot connect a vertex to another one closed.

i am just trying to make an easier block out of my model by combining different modeling methods looks fine now but when i turbosmooth the model it does not

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You can’t do that, as I explained. I can see what you’re trying to do, but I don’t understand why. What is it you’re trying to model?
TurboSmooth will definitely not work as expected in those circumstances.

Max 4.2 through 2013.
XP-64 (SP2)
NVidia 9800GTX-512 (Driver 266.58).
Core 2 Quad Q6600 2.4GHz, 8Gb Ram, DX9.0c.

i see what i am doing wrong thanks Steve and everyone else