Cool, now that we understand you're talking about your PWAD. (Your original post said: "How do I extract ... a ... texture ... and put it into Doom2.wad", and that caused the confusion.)
It's not the Doom2.wad I want to screw around with, firstly. It's for a level I am doing.
First you need to export your sky graphic from the HR wad. Open the HR wad and press your F7 key to open up the lump manager menu. Select Export LUMP to a FILE to open up a new menu. In the left column, scroll down (or use the text window to type the name of your HR sky patch) to the sky -- say RSKY1. Press the LMP button, and when prompted, select the folder into which you want the file to be saved.
Secondly, I know what to do, but how do I do it?
Next, open up your PWAD and press your F7 key. Select the Merge-Import Multiple FILES .... button. Select Add Lump Files and browse to the folder in which you had saved your sky graphic as a lump (it would be called rsky1.lmp or whatever the lump name is in HR, unless you had renamed it during the export). Next, select Open Target File (your PWAD), and then Save ALL Files. If your lump name is the same as a DooM2 lump name (e.g., RSKY1), you're all set. If not, then you'll need to insert a PP_START before and PP_END after your sky graphic lump (use Insert Target).
If your lump name is not a DooM2 lump name you'll need to insert the sky patch either into an existing sky texture (SKY1, SKY2, or SKY3), or create a new sky texture (if your PWAD is for a source port that supports additional skies). If you're not using a source port, your best bet is to make sure your lump name is the same as the relevant DooM2 lump name (i.e., RSKY1, RSKY2, or RSKY3). If you insist on using a different lump name, then you'll have to go through the TEXTUREx Name Edit in the lump management menu (F7). I won't go into the explanation here.
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