You need Morgh´s, it is a very useful tool that allows you to edit everything on items, troops and a few more. (https://forums.taleworlds.com/index.php/topic,105928.0.html)
With that tool you must open Gekokujo, find the yari you wish to change and, depending on what you mean by "crouching lance" do the next:
-Couched lance as in lowering the yari while riding at full gallop so you can skewer the enemy: Take a look at the section called "Flags", it is to the left followed by "Capabilities" and "Modifiers". In that section you must find an entry called "covers head/couchable". You will see a small empty square by its side, just tick it. Click "Update item # number". Do this for each item you wish and then click "Save Changes".
-Crouched lance as in crouching while on foot and bracing it to take an enemy cavalry charge. Once again look at "Flags" and tick "is_pike (WFAS)", it is near the bottom. The rest is exactly the same.