Hi Taxman, additional forces will deploy upon war entry. In late 1940 there is now a popup message which will let you know the location of these forces, which include the forces from the base game near Vybourg (variable strength based on winter war or not), the Garrison at Petsamo, as well as an Infantry Corps, Division and HQ at Joenseau, as historically the Finnish immediately marched Eastward into Soviet territory.
Finland can also receive additional units through the course of the war such as a Tank Division, Engineers, Anti-Tank and Artillery, as all are available via separate decisions (some conditional based on territorial gains or losses), in addition to possibility of German reinforcements sent to Northern Finland through Sweden (If Norway is surrendered, and Sweden is Axis leaning to permit rail transport).
Finland will also have the decision to sign the Tripartite Pact or not, which shields them from the other Allies. There is a massive Swedish penalty if the Western Allies DOW Finland, so it is really put in the Finnish hands whether to go to war with all countries or just fight their own continuation war.
Finland should be able to achieve Infantry Weapons I by the time they enter the war, which is desired TBH. In the newest version (not yet released) Finland will also receive 50 MPP the turn before war entry (as there is always that one turn delay) which will help in strengthening the deployed forces if necessary.