Welcome to the forums man!
Now, anyone please correct me if I'm wrong, but I do not think that the first question of yours is possible.
But not to say that you can't make enemies spawn, or appear, in a specific spot of your choosing upon the players entrance to a location.
You can use a Triggerzone for that, so say you wanted an enemy to appear behind you as you entered a hallway, just set up a Triggerzone, right click the zone, set its Main AI script to "plrinzoneactivateused", and in the If Used box 2 down from Main, put the name of the enemy you want to appear. Also, you'll have to place your enemy in the level where you want it to spawn (if its behind the player, then you'd have the enemy placed in the hallway before the Triggerzone. And you'll want to make sure the enemy has the same name, as the name you specified in the Triggerzone If Used section by also right clicking on it. Also, make sure the box Spawn At Start? in the enemies properties is set to No.
Of course, that is however a "pre-determined" starting point, but it adds a little variety.
Really if I'm wrong, someone please say, but I'm pretty sure the actual idea your asking, of a free-roaming character, is not possible.
For the second question, I'd just like to point out a fantastic program called "Signs IV" which is a big plus when it comes to things such as sign making, and others like segments and texts.
http://www.fpsconline.com/
You can take a look at it there (also, unfortunately, you'll have to wait for them to put it back up on sale, but it is an indespensable tool, definitely worth it).
Also, there is a free program you can download called Firma! Which can be found down the list a little there on that website page (an amazing website as well for FPSC media) (oh and also check out fpsfree.com).
http://fps-files.com/index.php?site=files&cat=13
With Firma you can not only make segments, but take the default FPSC signs, like for example the "Danger" sign you pointed out, and put your own texture on instead.
Its a very intuitive program, and comes with a manual showing how exactly to add your own images for the signs.
I hope I've been of some help to you.
Once again I'd just like to welcome you, and wish the greatest for you in your adventures of game making that lay at your feet before you