Guardsman93

Having trooped several times outside of my home Garrison, I can assure you it's definitely possible and invariably you'll be welcomed.  (I've also trooped as RL outside of the UK and had exactly the same, good experience.)  If you know that you're planning on visiting a location with enough time beforehand, I would suggest signing up to the garrison forum for the area that you plan to visit.  There, you'll be given visiting trooper (or similar) status and will be able to view their troops.  
 
Generally you sign up with them in the same way as you would you're home garrison, if in doubt how they list their signup, reach out in their Introductions forum.  
 
Then generally what happens if you'll let your home garrison know that you've signed up for a visiting troop and it'll be added to your service.  It's great fun and a brilliant way to get to know people.
 
We're all generally a friendly bunch anyway. 

Checking in on behalf of several of the Canadian Garrisons here; we're always happy to have visitors from other garrisons (except during COVID-19 lockdowns, of course).  Even if we don't have a troop going on, drinks, dinner, or other socializing can usually be arranged!
 
Two things to consider when crossing borders:  First is that volunteering may be construed as "working" in another country.  You may want to consider making a quiet inquiry with the immigration authorities before bringing your kit over.  Second; remember that if you are bringing a "toy blaster" across a border, it will likely be treated as if it were a real firearm. If you MUST bring it, pack it as if it were real, bring a photo of yourself in costume (for context), and refer to it at all times as a "toy blaster".  Don't use the word "prop" as some troopers have told me that there are places that have strong regulations and duties to be paid on "props", and never, ever, EVER refer to it as a "gun."
 
I, too, have a goal of visiting other garrisons, outposts, and squads, so perhaps we'll meet some day!  I wish you safe travels!

As above, if you're travelling overseas reach out to the local garrison.  Even if you can't troop with them they'll likely invite you along to meet and greet.
Take some merch to trade with you and you will always be extra welcome 

Anything is achieveable.