How can i access properties of classes that are not implemented via  the uses-clause, just knowing their names ( by string)?

You have to use the TypInfo unit.
Simple properties, like strings and integer, can be accessed in the
following manner:

-------------------------------------------------------------
Uses TypInfo;

procedure AlterProp(AName, APropName, AValue:string);
var  i : integer;
     C : TComponent;
begin
    // Run through all Components to find the right Component
    for i:=0 to Form1.Componentcount-1 do
    begin
        C := Form1.Components[i];
        if (C.Name = AName) then
        begin
            SetPropValue(C,APropName, AValue);
       end;
    end;
end;
-------------------------------------------------------------

Now there are also some Properties like Font. How do i reach those
sub-properties?
Here is a solution to that:

-------------------------------------------------------------
Uses TypInfo;

procedure AlterFontColor(AName:string; AColor:TColor);
var  i : integer;
     C : TComponent;
     AObj : TObject;
begin
    // Run through all Components to find the right Component
    for i:=0 to Form1.Componentcount-1 do
    begin
        C := Form1.Components[i];
        if (C.Name = AName) then
        begin
            AObj := GetObjectProp(C,'Font');
            SetPropValue(AObj,'Color',AColor);
        end;
    end;
end;
-------------------------------------------------------------

And finally you have many indexed properties like TStrings or stuff.
Now how do i reach those indexed properties?
All indexed properties are stored in TCollection-Objects. So you
have to Typecast them like in the following function:

-------------------------------------------------------------
Uses TypInfo;

procedure AlterIndexObject(ACompName, APropName:string; ACaption: string);
var  i : integer;
     C : TComponent;
     ACollectionItem, AObj : TObject;
begin
    // Run through all Components to find the right Component
    for i:=0 to Form1.Componentcount-1 do
    begin
        C := Form1.Components[i];
        if (C.Name = ACompName) then
        begin
            AObj:= GetObjectProp(C,APropName);
            ACollectionItem := TCollection(AObj).Items[0];
            SetPropValue(ACollectionItem, 'Caption', ACaption);
        end;
    end;
end;
-------------------------------------------------------------

There are many possibilities for this functions to use in your projects.
Mainly there are usefull for language-changings or skin-components.
The possibilities are endless.
I hope my article was somewhat usefull for you.
I like to share knowledge with other developers.

keep on coding :-)

Jürgen Sommer