With extra training many things become possible
To gain training in a prestige class two conditions must exist 1.)The character must meet all the requirements for the class that he/she wants to become 2.) The character must be abel to recieve training in that class.
To gaining a level in a prestige class that the character has fulfilled both conditions for. The character must wait until he/she has enough experience to gain a level in his/her normal class. The prestige class level can then be gained in place of the characters normal class. So the character can ether gain a level in his/her normal class or gain a level in his/her chosen prestige class.
[Example: A level 8 Thief named (Yeoman) wishes to become a Shadow Stalker. Yeoman meets both the Training and Class’s ability requirements. When Yeoman has earned enough experience to become a Level 9 Thief he can instead become a level 8 Thief with 1 level in Shadow Stalker.]