I have been trying to upgrade from 5.1 to version 5.4 for a while now.
(the problems seem to have stemmed from running an upgrade under a different schema to the initial installed schema, possibly a long time ago, but only just realised this, so too late for a restore).

I've got 2 schemas: 'dbo' and another one, I shall call 'nigel' schema.

I have therefore been trying to fix this by copying all the following:
tables,
functions,
views and
Stored Procedures
from the old 'nigel' schema and recreating to the 'dbo schema. Since doing this, the site still runs ok and so I believed the next upgrade should go ok too.

However, upgrading from 5.1.1 to 5.4.4. resulted in more error messages - as before:
e.g.  "Cannot find the object ’AddContentItem’ because it does not exist or….“

I checked under Programability >> Stored Procedures and they do not exist.
But they do exist in the development database which is running version 6.4.

My question is this: can I simply copy the missing objects such as:
tables, functions, views and Stored Procedures
from the development site which is running a higher version (6.4) and place these in my 5.1 (live) site prior to the next upgrade to 5.4?
Or will this cause issues?

Other than that, is there a better way around this?
perhaps by installing a new 5.1 installation on another development site and grabbing all the objects required from here?

Here's a breakdown of the number of objects:
DBO SCHEMA (5.1)           NIGEL SCHEMA (5.1) 
tables         1443                            128
functions      10                                 10
views            30                                 15
Stored Procedures 783               744

Correction
(sorry, 1443 was a typo error), should be:

DBO SCHEMA (5.1)
tables         143