Change this line in the Transportation rule:-

If the target Territory is controlled by another Country, it cannot be moved to.

To:-

If the target Territory is controlled by another Country, this is called Invasion Transportation (or simply Invasion), which can only be done from an adjacent Territory and costs the same as Hostile Transportation. Invasion Transportation lasts an indefinite number of periods (use ‘?’ to represent the number of periods in the “In Transit” section). In order to make an attack post at that Territory, the invading Country must pay for a period of Invasion Transportation. If the defending Country loses control of the Territory, the attacker may reduce the number of periods to 0 and thereby complete the Invasion. At any time, the invading Country may choose to reduce the number of units in the Invasion Transportation; these units then get added back to that Country’s population presence in the origin Territory. As long as the origin and target Territories are the same, multiple Invasion Transportation entries belonging to the same Country may be combined, adding the number of population units being moved together into one entry.

Also in Transportation, lower the cost of Frontier Transportation to twice Basic Transportation, and lower Hostile Transportation to five times Basic Transportation.

And add this to the Conflict rule:-

In the case of an attack method used in the course of an Invasion, the population units being moved in that Invasion Transportation is considered the attacking presence, and the population units in the target Territory counts as both the defending presence as well as the native population.

A Country performing an Invasion may also use a special type of Attack Method, Invade, which is effective against all Government types. If the attacking presence is greater than half the defending presence, the defending Country loses control of the Territory, which then becomes disputed.