Extending vector grid in QGIS

As an idea you can try using the "Translate" geoalgorithm from the QGIS's Toolbox.

'Offset distance (x-axis)' and 'Offset distance (y-axis)' will be a combination of two values from a set of (len, 0, -len) values, where len is the a length/width of your grid layer.

('len', 0)
('len', '-len')
('len', 'len')
(0, 'len')
(0, '-len')
('-len', 'len')
('-len', 0)
('-len', '-len')

So, it is like a Moore neighborhood, where there is a central cell and there are eight cells which surround it.

Here is an example:

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References:

• Docs » QGIS User Guide » 23.1.15.78. Translate