Manually Declare Global Variable Is it possible to manually declare a global variable that is inside of a function. The only current way I know how to do so is to declare that variable within the setup() function or simply outside of any function, but I was hoping there was a way to manually declare a variable as global so that I may call one variable from within one function in another function.
You need to research 'Scope'.

Your variable has to be at the same level as all functions that will use it.  So at a simple level the answer has to be no.

int variableOne = 0;
void setup ()
{
  variableOne = 1;
}
void loop ()
{
  variableOne = 2;
}


The above code will work, but if you move int variableOne = 0; inside either function then it won't work, because the variable will not be in scope in the other function.

You can use parameters to functions.  

void AFunction (const int&amp; variable);


Will let you pass a single integer value to the function from the parent.  You can't change that value, because of the const qualifier.  If you remove the const then the &amp; means pass the parameter by reference, which means you are passing in a reference (pointer) to the variable in the parent function and you can change it.  A better way to change the parent's value is to return a value from the call.

int AFunction (const int&amp; variable);


So you could use this like this:

int AFunction (const int&amp; variable)
{
  return (++variable);
}
void loop (void)
{
  int value = 0;
  value = AFunction(value);
  // value now == 1.
}


You could use a Singleton class, but this is hardcore and I'm not sure you would see any real benefit on an Arduino.  A class is a grouping of related function and variables that you see in libraries and a Singleton class means there will only ever be one during the lifetime of the program.  In this way you can declare an instance of the class (i.e. CSingleInstanceClass onlyOne;) and then retrieve/set data from the setup and loop function, i.e.

void setup ()
{
  CSingleInstanceClass onlyOne;  // Single instance class
  onlyOne.SetVariableOne (1);
}
void loop ()
{
  CSingleInstanceClass onlyOne;  // Single instance class
  Serial.println(onlyOne.GetVariableOne());// Prints 1
}


Hope that helps.