Adding a gallon in the secondary?

I have a 5 gallon primary (plastic) and a 6 gallon secondary (glass). My question is this ... if I use more malt in my wort can I add a gallon of water when I rack to the secondary to increase the volume of my final beer without seriously affecting the final flavor? My desire is to make enough in a batch to keg close to 5 G and still have a gallon or so to bottle (for friends and family.) Is this a really bad idea?

you could, if you are 100% sure you have sterile water going in. Personally I wouldn't risk it though.

You can always bottle out of your keg for your friends and family

Yeah, it seems like trying to get something for nothing, but when beer is involved the idea of an extra gallon for the same amount of work is really appealing. I guess I should just bottle from the keg as you suggest.

Diluting the flavor by 20% can change a good brew into BMC. This isn't always the case, my Bent Rod Rye started by diluting Hop Rod Rye to see what it would taste like as a session ale. BRR is about 2/3's of what HRR is. With Poor Richard's Ale, I found I could mix half ale & half carbonated water and still have something very drinkable,so next summer, I'll probably brew a half recipe and ferment as a lawnmower ale.

If you want to bottle & keg in the future just recalculate your recipes for 6 gallons.

David's right, although you'll need a bigger bucket for primary fermenation. Spend the $10 or whatever for a bigger one, you'll more than make up for it by being able to brew 6 gallons for the same work as 5.

If you do decide to go this route (which isn't the worst idea in the world if you calculate your recipe for a 6-gallon batch), just be absolutely sure to boil the water for at least 15 minutes, then cool it, before adding it...in order to deoxygenate it. Otherwise you'll introduce oxygen to the beer and it will turn to shyte faster.