Can I put a different brand lens onto my Dslr?

Nikon or Canon, no.

Third-party brand, yes. Among them, the best would be Tokina followed by Sigma and far behind is Tamron. Don't even dare touch other brands.

There are also a few brands that are better than the original. Those are the hard to pronounce names that come without zoom and autofocus. These are Carl Zeiss (who also makes lenses for Sony), Schneider Kreuznach (I hope I got that right) and Voigtlander.

Although you should be able to find an adapter for Nikon F or Canon EF to Sony A, the disadvantages will far outweigh any minimal benefits you might realize. Its just a bad idea.

If you're wanting to add to your lens collection for your Sony A700 then start looking for Minolta Maxxum AF lenses in the "Used Dept." at B&H or KEH or Adorama or on eBay. Case in point: A Minolta Maxxum AF 70-210mm f4 constant aperture zoom that will auto focus and meter on your A700 can be found on eBay, offered by "Top Rated" sellers, for around $200.00. I own and use the older manual focus version of this lens and know from research that the internals of both versions are identical. The only modification was to the lens barrel which was redesigned for the auto focus mechanics. Here are examples of how this zoom performed under some challenging conditions:

http://www.flickr.com/photos/drifter45h/7308480918...

http://www.flickr.com/photos/drifter45h/6003606300...

http://www.flickr.com/photos/drifter45h/7308480458...

So don't waste your time and money on adapting a different brand of lens to your A700.

B&H http://www.bhphotovideo.com/

Adorama http://www.adorama.com/

KEH http://www.keh.com/

Yes, you can put a different brand lens on you Sony DSLR. All you need is an adapter such as this one.

http://www.amazon.com/s/ref=nb_sb_ss_i_1_13?url=se...

As to whether that is a good idea, is a different question.

No. Even with an adapter (if you can find one) you probably won't get full metering and on some lenses not even aperture control.