I wouldn't. You run the risk of getting rocks that will infect your tank, or alter your water chemistry too. Run-off from pesitcides, chemicals, road treatments, etc is a major concern in taking rocks and plants from a local stream.
If you want to add rocks, I would make sure you boil the rocks for about an hour and then QT
them, just to be safe. I wouldn't add plants though, or fish...too risky.
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