Depends on what you're trying to convert.
First, and most importantly, make sure you get the permission of the original addon creator, unless it explicitly states in the addon's EULA that you're free to modify and distribute the addon.
Configs are generally going to have to be completely rewritten, as a lot of inheritance probably won't work (there are classes in OFP that aren't in ArmA, and visa versa, a lot of the classes that are in both games are a lot different)
If you want to convert infantry, you're probably better redoing the models from scratch (too much has changed about infantry from OFP to ArmA to make it worthwhile, it'll take a lot of time to convert soldiers from OFP to ArmA and the end results won't be very good)
Weapon models are as simple as just applying one of the ArmA weapon model.cfg files to the model (even this isn't necessary, but the selections won't work properly if you don't). For extra kudos you should add the magazine to a selection named "magazine". You might also want to modify the textures and create material files for better compatibility.
Vehicles and Aircraft are done in a similar manner. I'm not an expert with vehicles though, so I'm afraid I can't really help you. I'd advise looking at one of the sample models released and modifying the selections on your model to match them, then use the same model.cfg.
ArmA weapon model.cfg:
CODE
class CfgSkeletons
{
class Default;
class Weapon: Default
{
isDiscrete = 0;
skeletonInherit = "Default";
skeletonBones[]=
{
"magazine",
"",
"ammo_belt",
"",
"ammo_belt_bottom",
""
};
};
class WeaponRevolving: Weapon
{
skeletonInherit = "Default";
skeletonBones[]=
{
"buben",
""
};
};
};
class CfgModels
{
class Default;
class Weapon: Default
{
sections[]=
{
"zasleh"
};
skeletonName="Weapon";
class Animations
{
class magazine_hide
{
type="hide";
source="reloadMagazine";
selection="magazine";
minValue=0.200000;
maxValue=0.250000;
hideValue=0.100000;
};
class ammobelt_hide
{
type="hide";
source="isEmpty";
selection="ammo_belt";
minValue=0.000000;
maxValue=1.000000;
hideValue=1.000000;
};
class ammobelt_bottom_hide
{
type="hide";
source="isEmpty";
selection="ammo_belt_bottom";
minValue=0.000000;
maxValue=1.000000;
hideValue=1.000000;
};
};
};
class WeaponRevolving: Weapon
{
skeletonName="WeaponRevolving";
class Animations
{
class Revolving
{
type="rotation";
source="revolving";
selection="buben";
axis="osa_buben";
memory="true";
animPeriod=0;
angle0=0;
angle1="rad -360";
};
};
};
class MyWeaponsModel: Weapon
{
};
};