Custom Insert and Modify

I have 3 tables
1- Articles
id | title
0 | Artic #1
1 | Artic # 2
2 | Artic # 3
3 | Artic # 4

2- Categories
id | cat
0 | Cat #1
1 | Cat # 2

3- Relations
id | cat | artic
0 | 0 | 1
1 | 0 | 3
2 | 1 | 0
3 | 1 | 1

Its posible to make insert and modify with Data Assist?

Do you meen insert and modify at the same time if so yes you can but not with the wizard you would need to call them from the server behavoir panel

As you can see from the table RELATIONS IDs join the other two tables ...
I need a page to insert records, that having selected the category, let me insert N items. The relationships between categories and articles will insert in the table relations.

I don't see anything in your descriptions that would not be able to be done with DataAssist, like Sades noted though you will need to modify the pages after they are created by the wizard and apply the other server behaviors. Sounds like you will need to make use of multi insert, multi update and manage relational table server behaviors.