[SalesForce] How to push fields on packaged objects with SFDX

I'll answer as I did find some kind of workaround

Those steps allow me to work DX-style with Managed Package, Namespace and Person Account

1 Create scratch org WITHOUT Namespace, using flag -n, --nonamespace

sfdx force:org:create -f config/project-scratch-def.json -a ScratchNONameSpace -s -n

2 First thing is to pull Account.Business_Account, which works as expected

$ sfdx force:source:status -r

=== Source Status
STATE       FULL NAME                 TYPE        PROJECT PATH
Remote Add  Account.Business_Account  RecordType

$ sfdx force:source:pull

=== Pulled SourceSTATE  FULL NAME                 TYPE        PROJECT PATH
Add    Account.Business_Account  RecordType  force-app/main/default/objects/Account/recordTypes/Business_Account.recordType-meta.xml

3 Create scratch org WITH Namespace

sfdx force:org:create -f config/project-scratch-def.json -a ScratchWithNameSpace -s

4 Check status, you'll see conflicts

$ sfdx force:source:status
=== Source Status
STATE                      FULL NAME                 TYPE        PROJECT PATH

Local Add (Conflict)       Account.Business_Account  RecordType  force-app/main/default/objects/Account/recordTypes/Business_Account.recordType-meta.xml
Remote Changed (Conflict)  Account.Business_Account  RecordType  force-app/main/default/objects/Account/recordTypes/Business_Account.recordType-meta.xml

5 Push your code using flag -f, --forceoverwrite

$ sfdx force:source:push -f
=== Pushed Source
STATE  FULL NAME                 TYPE        PROJECT PATH

Add    Account.Business_Account  RecordType  force-app/main/default/objects/Account/recordTypes/Business_Account.recordType-meta.xml

$ sfdx force:source:status
=== Source Status
No results found

After that you would be able to push/pull without issues.

You have a space in your file path, so the CLI thinks you're trying to add multiple paths. Make sure you either quote:

"/Users/salesforceDeveloper/Desktop/SFDC Project/Prod Migration/force-app/main/default/objects"

Or backslash escape:

/Users/salesforceDeveloper/Desktop/SFDC\ Project/Prod\ Migration/force-app/main/default/objects

Or ideally, move the folder to a place without spaces. They tend to cause problems wherever you go. I recommend something like /workspace/salesforce/migration or something like that.

To verify only, use -c as a parameter:

sfdx force:source:deploy -p force-app/ -c -w 100 -l RunLocalTests -u targetOrg

I also recommend a wait time (-w 100) so that you can get feedback when the deployment is finished. If you don't use this parameter, you need to run force:source:deploy:report later, which is usually just annoying.