Apart from attribute increases at character creation.... Is there anyway to increase your encumbrance level?

Buy a backpack or similar gear.

There are various backpacks and other similar items (from utility belts adding +1 to military backpack adding +6), as well as some armor mods (Repulsor-assisted Lifting, +5 to +7) and probably other things.

There's also the Burly talent

Quote

Reduce any wielded weapon’s Cumbersome quality and Encumbrance rating by ranks in Burly to a minimum of 1.

Only works when wielding a weapon and not when it's in a holster or your bag, but still handy.

Dont forget to use five or six backpacks! That'll let your character carry a walker.

10 minutes ago, Desslok said:

Dont forget to use five or six backpacks! That'll let your character carry a walker.

/s

Yup, or one bag of holding

Unencumbered...

JFYI: characters aren't going to be carrying a golf-bag full of swords, a la classic D&D or GURPS. 😉 Encumbrance primarily focuses on "are they strong enough to pick up and use this weapon," not "how do I carry a platoon's worth of weapons on one person."

In early campaigns I had players try to "loot the bodies" and I told them that, unless they come with a convincing method to carry all this stolen equipment, they're going to have to leave it behind.