Call web service action using a system account/farm account instead of Workflow constant

[Manisha]

Hi,

I've a requirement to copy files from different sites (under different site collections) to my site. I'm using call webservice action and with copy.asmx, my files are getting copied. All of this is working with the site admin account.

My challenge begins when I try to use a different account to run the web service. I don't have certainty of the source file location. Users can enter any location in the tenant and most likely my account may not have access, failing which the copy fails. 

I can't go ahead and give site collection admin rights to this account for all the site collections (although it would have solved my problems) as it would introduce a risk with two site collection admins.

So, question(s)

• Is there a way I can invoke the web service under the context of the 'system account' aka the farm account, without using manage credentials? 
• Without having to grant permissions for the workflow account granularly across thousands of subsites, is there a way to accomplish my requirement?

Thanks!