Opening in a separate file is probably the best way to do it.
That way you can check all attributes (pen colours, surface materials, layers, etc.) so it appears exactly as you want before you copy it into your model.
That way you won't get attribute pollution.
I am not sure about STL file, but other model formats may include their own attributes for layers, surfaces, etc.
And they will create these new attributes when you open them.
This is attribute pollution.
You can merge directly into your open project - FILE menu > File Special > Merge from File.
But you may not have control of any new attributes being created, so your file will pollute with new attributes that are not part of your template defaults.
But if you are not concerned about that, just merge.
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