Or you could download JassNewGenPack over
here.
This editor has the capability of removing the 8000 doodad limit and also,
gives you the ability to specify which terrain tile should have which pathing (walkable, buildable and flyable).
It is highly recommended to use this editor since it adds a lot of usefull funtionality to the standard world editor.
If you want to change your tile pathing in JassNewGenPack all you have to do is go to:
UMSWE->Customize Tile Pathability
This will show all the tiles which you can then select and change it's pathing to whatever you like.
If you really don't want to use JassNewGenPack for some odd reason, then an alternative would be manually creating a pathing for your tiles.
Here is a tutorial on how to create pathing maps:
http://world-editor-tutorials.thehelper.net/pathmaps.php
You'll need:
Image Editing Software
an MPQ Editor (In order to edit a existing pathing map)
I hope this helps.